This invention relates to a composite package for containing pressurized fluids and, more particularly, relates to a composite package having improved stability and eversion resistance.
Biaxially oriented thermoplastic bottles are receiving much attention as candidates to replace glass bottles presently being used to contain carbonated soft drinks, beer, aerosols, and the like. The attractiveness of thermoplastic bottles lies in their lower weight and general breakage resistance. On a per-pound basis, however, thermoplastics are more expensive than glass and it is necessary to minimize the weight of thermoplastic bottles if they are to be economically competitive with glass bottles. Consequently, the emphasis is to make thin-walled thermoplastic bottles.
Self-standing, thin-walled, thermoplastic bottles, when pressurized, experience severe forces which tend to cause the bottom to evert (i.e., assume a more hemispherical shape), causing the bottles to rock or tip over when placed on a flat surface. The art has suggested that the stability of such bottles can be improved by increasing the thickness of the bottle bottom, commonly in conjunction with use of an eversion-resistant bottom design, or by use of composite packages wherein a support ring or cup is attached to the bottle. In general, however, it is preferred to minimize bottle thickness and the more promising method for achieving stability is to employ a convex-bottom bottle with an inexpensive support cup.
Of the various thermoplastic materials, polyethylene terephthalate is regarded as one of the more promising candidates due to its high strength, impact resistance, and ability to hold liquids under pressure when biaxially oriented. Biaxially oriented polyethylene terephthalate bottles, however, tend to have an area of localized spherulitic crystallinity at their extreme bottom as explained hereinafter. This crystalline area is fragile, relative to other portions of the bottle, and may rupture when struck a sharp blow. For instance, the bottle may rupture if it lands on the crystalline area when dropped.
Polyethylene terephthalate has a marked tendency to crystallize when cooled from the molten state and it is extremely difficult, in a commercial process, to eliminate all traces of this crystallinity. Namely, when injection molded to form a bottle preform, an area of spherulitic crystallinity tends to form in portions of the preform which are adjacent to the mold gate. When such a closed end tubular preform is blow molded to form a bottle, the crystalline area is at the extreme bottom or nub of the closed end and is readily detectable due to its milky-white appearance.
Biaxially oriented bottles are readily made from these preforms using blow molding techniques well known in the art. In the finished bottles, the spherulitic crystallinity formed during preform manufacture is present at the extreme bottom (i.e., the nub) of the bottle. Thus, the bottom of polyethylene terephthalate bottles not only tends to evert under pressure, a problem common to thermoplastic bottles, but the nub is also particularly sensitive to rupture.